Solar water heater (SWH) is one of the applications of solar energy thermal technology that serves to produce hot air to fulfill household and industrial. The fluctuation of solar radiation intensity leads to research of SWH based on phase change material (PCM) active systems with outdoor experimentation that is unable to uncover the influence of the magnitude of input energy source against thermal characteristics are produced. The study was conducted with the aim of thermal energy storage characteristics of the SWH tanks during the filling process using heat flux variations. This experiment was done by indoor method using solar simulator which serves to supply constant heat flux on SWH-PCM. Solar simulator is mounted on the top of the collector. Solar Simulator is set in such a way that produces heat flux with a variation of 800 W/m2, 1000 W/m2, and 1200 W/m2. The water temperature Data in the tank is recorded. Testing is repeated by changing the flux heat variation. Data analysis is conducted to determine the accumulation of thermal energy storage stored in the SWH tank. Products research results that this research con heat flux of 1200 W/m2 and discharge 2 LPM. The accumulation of heat storage is 10, 07 MJ during the time of charging 98 minutes. About this can be used to predict when it is needed in the SWH heating process. When liquid heating parameters repent in assessing SWH performance.